<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var f=true
        if(f==true){
            var a=10;
        }
        function fn(){
            var b=20;
            c=30;

        }
        fn()
        console.log(a) //a=10
        console.log(b)//未定义
        console.log(c)
    </script>
</body>
</html>